The present invention relates generally to a system for detecting foreign particles or voids in plastic material, and more particularly to a method and technique for remote and nondestructive detection of defects such as voids, contaminants, and flaws etc., inside nonconducting thermoplastic materials, such as polyethylene, PVC, or any far-infrared transparent media. The disclosed detection scheme for defects in plastics utilizes a far-infrared (FIR) laser emitting in the submillimeter wave region at which most plastic materials become transparent as if the visible light passes through glass. Either a single or a cluster of defects inside a solid dielectric medium will scatter the laser beam in a predictable manner in predominantly either the forward or the backward direction depending on the nature of the defects. The captured scattered radiation pattern can be related to the size, shape, and the type of defects. This disclosure provides the basic method and technique for flaw detection in solid dielectrics, which are opaque to the light in the visible and the near infrared (.lambda.&lt;70 microns) region.
X-rays and ultrasonic waves have been often used to image and to detect defects and structural characteristics of opaque materials. For a class of materials such as plastics, these methods are not suitable. X-rays methods do not provide sufficient contrast ratio between the defects such as voids and micro-cracks and the medium in order to establish a clear image. Ultrasonic waves on the other hand can not propagate very deeply into plastic materials because of excessive loss of the media at the acoustic frequencies.
Plastic materials are increasingly used in the present-day consumer and industrial products. There is a need for a reliable, nondestructive method to inspect and to characterize the quality of these products. Because of inadequate quality control, plstic materials have not been widely used by the automotive, marine and building industries. This invention discloses a method by which the defects as small as a few microns in size can be reliably detected inside an opaque plastic material without destroying the object. This method utilizes a far-infrared laser scattering process by which the scattered light as a result of a small localized difference in the refractive index inside the plastic medium produces a characteristic pattern and is completely deviated from the primary laser beam in the forward direction. From the measurements of the scattered light, it has been demonstrated that the character of the defects can be recognized.
Laser scattering techniques have been used frequently to determine, for example, the surface (Ref. 1, P. K. Cheo and J. Renau, J. Opt. Soc. Am. 59, 821, 1969) the aerosol (Ref. 2, F. B. Fernald, et al., Opt. Quant. Elect. 7, 141, 1975) and others. However, these techniques are different from this disclosure, in that, they utilize only the reflection or so-called back-scattering of light. In these cases, the optical absorptivity of the medium is not crucial to the applicability of these detection methods. To detect small defects in plastic materials, it is necessary to select laser wavelengths such that not only the laser can penetrate the medium but also can provide adequate resolution to recognize the character of the defects. In other words, the method disclosed herein requires specific laser wavelengths which fall in the range from 70 microns to 2000 microns.
The absorption coefficient of most plastics such as polyethylene, polypropylene, teflon, etc., decreases montonically with increasing laser wavelength. On the other hand, the scattered laser power, in general, decreases with increasing wavelength for the size of defects in the proximity of one optical wavelength. For these reasons the choice of laser wavelength is an important consideration for the design of a detection system which is required to detect defects having a specific size range of interest.